Biomechanics of spinal deformity€¦ · Principles of Deformity Correction Many factors must be...

Pathological spinal deformation, either in an acute orchronic form, is usually a result of at least one unstablemotion segment. For the deformity to progress, patholog-ical stressors must be applied to the spine, although non-pathological stressors applied to an already deformedspine may also cause further deformity. The spine surgeonis faced with significant challenges in formulating treat-ment strategies for both spinal deformities and deformityprogression. Objectives including curvature correction,prevention of further deformity, restoration of sagittal andcoronal balance, cosmetic optimization, and improvementand preservation of neurological function must be si-multaneously considered during treatment planning tooptimize successful outcomes.2 Spinal deformities arecomplex entities, often composed of more than one de-formation type with more than one attendant problem. Acomplex solution can be achieved by combining compo-nent strategies for each aspect of the complex problem.Awareness of the complexities of deformation develop-ment and progression is critical to the design of an ap-propriate management scheme. The surgeon must try tounderstand the forces at play in creating the deformity,which is necessary because during the correction theforces must be countered or reversed, thereby neutralizingthe pathological forces. Complete deformity correction isoften the goal, but it is not necessarily required to allevi-

ate symptoms and prevent further deformity or neuro-logical deficit. The goal of surgery, hence, should be theattainment and maintenance of a nonpathological relation-ship between the neural elements and their supporting andsurrounding osseous and soft-tissue structures.

SPINAL DEFORMITIES

Physical Principles and Kinematics

Forces applied to the spine can be broken down intocomponent vectors. A vector is defined as a force orient-ed in a fixed and well-defined direction in three-dimen-sional space. A force vector may act on a lever (momentarm), creating a bending moment. The bending momentapplied to a point in space causes rotation, or a tendencyto rotate, around an axis. This axis is termed the IAR. TheIAR acts as a pivot point or fulcrum around which flexionor extension occur.

The moment arm is a lever that extends from the IAR tothe position of application of force to the spine (Fig. 1).The bending moment (M) is defined as the product of theforce (F) applied to the lever arm and length of the leverarm (D) in the following formula: M = F � D. The bend-ing moment is effectively the torque applied by a circularforce. The magnitude of a circular force is the torque.Around each of the three axes of the cartesian coordinatesystem, translation or rotation can occur.

Therefore, six fundamental segmental movements ofthe spine along or around the IAR can occur: 1) rotationor translation around the long axis; 2) rotation or transla-

Neurosurg Focus 14 (1):Article 2, 2003, Click here to return to Table of Contents

Biomechanics of spinal deformity

RICHARD P. SCHLENK , M.D., ROBERT J. KOWALSKI , M.D., M.S., P.E.,AND EDWARD C. BENZEL , M.D.

Department of Neurosurgery, Cleveland Clinic Foundation, Cleveland, Ohio

The correction of spinal deformity may be achieved by a variety of methods, each of which has advantages and dis-advantages. The goals of spinal deformity surgery include reasonable correction of the curvature, prevention of furtherdeformation, improvement of sagittal and coronal balance, optimization of cosmetic issues, and restoration/pres-ervation of function. The failure to consider all these factors appropriately may result in a suboptimal outcome. Un-derstanding fundamental biomechanical principles involved in the formation, progression, and treatment of spinaldeformities is essential in the clinical decision-making process.

KEY WORDS • spinal deformity • biomechanics

Neurosurg. Focus / Volume 14 / January, 2003 1

Abbreviations used in this paper: CCJ = craniocervical junction;CSL = central sacral line; IAR = instantaneous axis of rotation;VB = vertebral body.

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tion around the coronal axis; 3) rotation or translationaround the sagittal axis of the spine; 4) translation alongthe long axis of the spine; 5) translation along the coronalaxis; and 6) translation along the sagittal axis of the spine(Fig. 2). Each movement may result in deformation in oneof two directions involving one or many spinal segmentsas a result of acute or chronically applied loads.

Rotational Deformation

A bending moment affects a spinal segment by the ap-plication of an eccentrically placed load (that is, eccentricto the IAR). Angular deformation of the spinal segmentmay result along one or both of the axially oriented axesfollowing the application of a bending moment. Such ro-tation can take the form of kyphosis (flexion rotationdeformation), lordosis (extension rotation deformation),

scoliosis (lateral bending rotation deformation), or a com-bination of these. Rotational deformations are manifes-tations of an asymmetrical load or a rotatory load (torque)applied to a spinal segment. Rotational deformationsaround an axially oriented axis (coronal or sagittal) canoccur at the level of the VB or intervertebral discspace.5,10,15,32 Segmental spinal rotatory deformation canalso occur around the long axis of the spine (Fig. 3). Theapplication of a rotatory or torsional load to the spine(either acutely due to trauma or chronically due to gradu-al deformity progression) can cause the spinal segmentsabove the unstable segment to rotate in a direction in op-position to that below the unstable segment.

Translational Deformation

Shearing, compression, or distraction of the spinal seg-ments may result in translational deformation, whichoccurs along an axis defined by the direction of the de-formation-creating force vector.5,15,32 Translational defor-mation may occur in any plane. Burst fractures are causedby oppositional translational forces of the upper and lowerendplates of a VB. Axially oriented translational defor-mities occur secondary to two parallel but noncoincidentopposed force vectors resulting in fracture dislocation orspondylolisthesis. Distraction deformation is uncommonand usually accompanies a significant flexion componentin the acute form. Distraction forces may be applied us-ing spinal traction or by excessive implant-induced dis-traction. Most spinal deformities, however, are a result ofmore than one type of deformation.

Categories of Spinal Deformity

Spinal deformities may be divided into three funda-mental categories: 1) sagittal plane; 2) coronal plane; and3) axial plane. Many deformities are composed of a com-bination of sagittal-, coronal-, and axial-plane components(Fig. 4). Degenerative lumbar scoliosis is a common de-formity resulting in both a rotational and a kyphotic com-ponent.

Prevention of Deformity Progression

Excessive forces affecting the spine and/or nonpatho-logical stressors affecting an already deformed spine are

R. P. Schlenk, R. J. Kowalski, and E. C. Benzel

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Fig. 1. A: A force vector in three-dimensional space. B: If aforce (F) is applied at a distance (d) from a fulcrum (IAR), a bend-ing moment (M) is created.

Fig. 2. The six fundamental segmental movements, or types of deformation, of the spine along or around the IAR are:1) rotation or translation around the long axis (A); 2) rotation or translation around the coronal axis (B); 3) rotation ortranslation around the sagittal axis (C); 4) translation along the long axis (A); 5) translation along the coronal axis (B);and 6) translation along the sagittal axis of the spine (C).


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required to create a deformation or for it to propagate. Aworking understanding of the neutral axis, Cobb angle,and the radius of the curvature is important in the deci-sion-making process when managing cases of spinal de-formity.

Prevention of kyphotic deformity progression requiresknowledge of the neutral axis (load-bearing axis). Place-ment of strut grafts well ventral to the neutral axis helpsprevent further kyphotic deformity (Fig. 5). This may,however, require the use of longer struts. Likewise, dorsalconstructs must be placed well behind the neutral axis toprevent the progression of kyphosis.

The Cobb angle is measured from rostral and caudalneutral vertebrae associated with a curve. A neutral verte-bra is located in the transition zone between two curves.As the Cobb angle increases, an increased moment arm isapplied to the spine. The radius of the curve is importantto consider when measuring the Cobb angle. Segmentalangulations, when acute, may have similar angular mea-surements compared with less acutely segmental angula-tions. The radius of acute angular deformities is inherent-ly shorter than that of the same curve over more spinalsegments.

Principles of Deformity Correction

Many factors must be considered when attempting tocorrect a spinal deformity. The deformities are frequentlymultisegmental. Thus, strategies must be applied to mul-tiple spinal segments. A deformity that occurs along oraround an axis of the cartesian coordinate system oftenproduces another motion along or around another axis(that is, coupling phenomenon).

The load-bearing axis in both the sagittal and coronalplanes must also be considered. The load-bearing axis inthe healthy cervical and lumbar regions is located in thedorsal region of the VBs. Conversely, in the normal tho-racic spine, it is located in the ventral region. The loadbearing neutral axis is usually in the region of the middlecolumn (Fig. 6). The neutral axis is shifted ventrally withflexion and dorsally in extension.

Sagittal balance of the spine must be considered.16–18,21,28

In the standing position, a plumb line dropped from the

mid-C-7 VB should fall in the region of the dorsal L5–S1interspace. A negative 2 to 4–cm space behind this regionis normal. As an individual ages, this line can usually beseen to move forward. A loss of balance may occur, withthe sagittal vertebral axis falling quite ventral to S-1 (Fig.7).17,18 The CSL is used to assess balance in the coronalplane and as well as in the assessment of scoliosis.1 TheCSL is one that is perpendicular to a line passing throughboth iliac crests, ascending rostrally in the line with thesacral spinous processes. The vertebrae bisected by thisline are termed stable vertebrae.

The length and location of the stabilizing construct arecritical. On one hand, implant length must be sufficient toapply the necessary bending moment to the spine. On theother hand, it must not be so long that it creates excessivespinal stiffness.

The apical and neutral vertebrae in the coronal and sag-ittal planes must be assessed. Compared with all otherintervertebral disc spaces in the curve, the apical vertebraeare those associated with the greatest segmental angula-tion at both its rostral and caudal disc interspaces (Fig. 8upper). The apical vertebra is typically located in the mid-portion and horizon of the curve. The neutral vertebra usu-ally has little or no angulation at its rostral and caudal discspaces and is the vertebra located between curves (Fig. 8lower). The apical and neutral vertebrae should be estab-lished on radiographs in the coronal and sagittal planes.An implant should not terminate at or near an apical ver-tebra, and should, in general, be long enough to extend tothe neutral vertebra. If a long moment arm is extended to,but not above, an apical vertebra, there exists a significant

Neurosurg. Focus / Volume 14 / January, 2003


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Fig. 3. A twisting of the spine around its long axis (A) can resultin a rotatory deformation around the axis (B). Curved arrowdepicts applied bending moment.

Fig. 4. Sagittal- (A), coronal- (B) and axial-plane (C) deformi-ties are the three fundamental deformations that contribute to allspinal deformities, either individually or in combination.


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risk for deformation progression (Fig. 9). The determina-tion of the apical vertebra is therefore a critical componentof the decision-making process.

The cervicothoracic and thoracolumbar regions areprone to deformity and deformity progression if the im-plants are placed to, but not beyond, these levels. Discspaces adjacent at the junctional zones are usually not par-allel to the ground in the standing position, thus applyingangular forces to the spine.

Several clinically relevant deformity classificationschemes have been developed. King, et al.,20 divided coro-nal-plane deformities into five categories for classificationof idiopathic scoliosis: Type I, a double concave curve inwhich the lumbar curve is larger and more rigid than thethoracic curve; Type II, a double concave curve in the tho-racic curve is more rigid than the lumbar curve; Type III,a thoracic curve; Type IV, a long thoracic curve that tiltsinto the curve; and Type V, a double thoracic curve thattilts into the concavity (Fig. 10). The management of thesedeformities depends on curvature type and other patient-specific characteristics. In the scheme described by Lenke,et al.,23 the position of the lumbar apical vertebra with re-spect to the center sacral line is strongly emphasized (Fig.11). The compensatory curve forms in response to the pri-mary curve’s reflexive attempt to achieve balance.

COMPONENT STRATEGIES FOR DEFORMITYPREVENTION AND CORRECTION

Complex deformities can be corrected using spinal im-

plant–induced forces along one axis or a combinationof the three axes of the cartesian coordinate system, bywhich the spine is brought to the implant (Fig. 12). Bend-ing moments applied in the sagittal plane are of three- or



Fig. 5. A: A fixed (old) spinal deformity caused by two contiguous VB fractures. The neutral axis is depicted by theblack line and the load-bearing axis by the gray line. Note that compensatory spinal curves have developed. B: Thisdeformity may be inappropriately managed by the placement of a ventral short-segment weight-bearing strut near theneutral axis, rather than ventral to the neutral axis near the load-bearing axis. This is problematic because the strut doesnot span the entire length of the injured and deformed portion of the spine, nor does it bridge the deformity from neutralvertebra to neutral vertebra. C: A longer strut may be required. The location of the neutral axis usually influences thisdecision-making process. In this case, however, the neutral axis diverges from the load-bearing axis. The ventral weight-bearing strut should not be placed behind the load-bearing axis, as is the case in B and C. Rather, it should be placed wellventral to the neutral axis and in line with the load-bearing axis. D: This may require an even longer construct thatextends well beyond the fractured levels. With such a deformity, an interbody graft that is positioned well ventral to theneutral axis and in line with the load-bearing axis and that extends to the neutral vertebra (that between kyphotic and lor-dotic curves) above and below the deformity neutralizes its negative effect. Deformity progression will thus be unlikely.

Fig. 6. The load-bearing axis (neutral axis; shaded region) (A)is generally considered to be located in the region of the middlecolumn of Denis. In extension, however, the load-bearing axis isshifted dorsally in the cervical spine (B). In flexion it is shifted ven-trally (C), and in lateral bending it is shifted laterally toward theconcavity of the curve.


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four-point bending or applied moment arm cantileverbeam types.

Three- or Four-Point Bending Force Application

Three-point bending force application consists of a ful-crum that directs a force vector contralateral to the direc-tion of the terminal force vectors (Fig. 13 upper left andright). In Harrington rod or universal spinal instrumenta-tion application, techniques involve three-point bendingforce application over multiple spinal segments. Dorsallydirected forces are applied at the upper and lower terminiof the construct and a ventrally directed force is applied atthe fulcrum. This is equal to the sum of the dorsally direct-ed forces. Three-point bending fixation may also be usedto correct a deformity near the termini of the construct, asopposed to the midportion (Fig. 13 lower left and right).These techniques may be used to correct deformities aswell as to prevent them.

Four-point bending force applications involve the load-ing of a long spinal segment with two transverse forces onone side and two on the other. The bending moment isconstant between the two intermediate points of force ap-plication. The bending moment peaks at the intermediatepoint of force application with three-point bending andtwo intermediate points of force application with four-point bending constructs.

Correction of Crossed-Rod Deformity

An established technique for the correction of thoracicand lumbar kyphotic deformities is the crossed-rod tech-nique (Fig. 14), which was first performed in the Har-rington rod distraction. With the subsequent use of Luquesublaminar wires, however, gradual reduction of kyphosiscan be achieved by sequentially bringing the spine towardthe rod4 (Fig. 15). With sequential hook insertion, univer-sal spinal instrumentation systems may also facilitate ap-plication of the crossed-rod technique.7

IN VIVO ALTERATION OF IMPLANTCONFIGURATION

Applied Moment Arm Cantilever Beam Force Application

For short-segment fixation, applied moment arm can-tilever beam constructs may be placed to reduce the cur-vature.13,22 Commonly applied in the thoracolumbar andlumbar regions for deformity correction in cases involvingburst and wedge compression fractures, these constructsrequire significant loads to be placed at the time of cor-rective surgery. Sagittal-plane applied moment arm can-tilever beam forces may be applied in either flexion orextension. An applied moment arm cantilever involvingdistraction and extension may be prone to failure if the



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Fig. 7. Sagittal balance. A: A spine in sagittal balance, with a generous but not excessive cervical lordosis, thoracickyphosis, and lumbar lordosis. B: A plumb line that is dropped from the mid-C-7 VB (SVA) in the standing positionfalls in the region of the lumbosacral pivot point (dorsal L5–S1 disc interspace). If this normal spinal contour is disturbedby a focal deformity, balance may be achieved by compensatory mechanisms. C: Note that if loss of lumbar lordosisis present, the SVA falls through the region of the sacral promontory. D: Significant imbalance, however, may be devel-op, resulting in the SVA falling at a significant distance ventral to the sacral promontory. The CSL is used to assess bal-ance in the coronal plane. E: The CSL is a line that is perpendicular to a line passing through both iliac crests, ascend-ing rostrally, in line with sacral spinous processes. The vertebrae bisected by this line are termed stable vertebrae(shadedvertebrae).


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implant is excessively loaded.25 Because interbody strutloading (for example, compression) significantly unloadsthe axial forces through the spinal implant, the risk ofhardware failure and progressive deformity is reduced.The procedure involving the sequential application of dis-traction forces (load bearing), decompression of the duralsac, placement of an interbody strut, and finally compres-

sion of the strut is deemed a load-bearing-to-load-sharingforce application.

Reduction of Short-Segment Parallelogram Deformity

Lateral translational deformities may be surgicallytreated using the technique of short-segment parallelo-gram reduction. This technique is used when treating lat-



Fig. 8. Upper: An apical vertebra occurs at the horizon or apex of a curve, either in the sagittal (A) or coronal plane(B). It is associated with adjacent disc interspaces that have the greatest segmental angulation (�) of all interspaces in thecurve, as depicted.Lower: The neutral vertebrae are located between curves, be it sagittal (A) or coronal (B). There islittle or no angulation at its rostral and caudal disc interspaces (�), as depicted.

Fig. 9. A: A long implant should usually not terminate at or near an apical vertebra. B: A longer implant may berequired. C: Spine deformation at the termini of the implant is to be expected if the implant terminates at the apex of acurve. D: This is also shown for correction of a scoliotic deformity. E: Note postoperative progression of deformity.The implant was placed up to, but not beyond, the apical vertebra.


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eral translational deformities. In it, a rigid cantilever beampedicle fixation is performed in the thoracic and lumbarregions (Fig. 16). Pedicle screws are first placed androds are applied to friction–glide tightness. Rod holders are

subsequently placed and a torque applied to both rodssimultaneously until fraction reduction is achieved. Rig-id cross-rod fixation maintains the achieved reduc-tion. Placement of structural bone graft is then followedby placing screws in compression mode to achieve loadsharing.

Crossed-Screw Fixation

The crossed-screw fixation technique requires an extra-cavitary approach.6 It can be used to alter sagittal- or coro-nal-plane abnormalities as an alternative to other short-segment fixation techniques. It requires two large VBscrews that bear axial loads and two ipsilateral smallerpedicle screws that attain reduction and prevent flexion orextension deformities. The near 90° screw toe–in angleprovides rigid cross-fixation (Fig. 17). Compression ordistraction of the pedicle screw addresses sagittal-planedeformities. Coronal-plane angles may be changed bymanipulating VB screw relationships. This technique inthe purest sense is seldom used. Its underlying principles,however, may often find utility.

In Vivo Implant Contouring

To achieve reduction of a spinal deformity, in vivoimplant contouring to alter segmental relationships isoften effective. After the placement of pedicle screws orhooks, the rods are fit to the shape of the spine. Subse-quent in vivo contouring of the rods, with the attachedspine, may be used to reduce the deformity. This tech-nique applies unknown and perhaps excessive forces tothe spine. Alteration of the relationship between the spineand the implant may occur, such that the screw or hook isovertightened or becomes loosened and then migrates.Sublaminar hooks may be inadvertently forced ventrally,impinging the thecal sac and spinal cord.



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Fig. 10. The King classification scheme for idiopathic scoliosis. A: Type I is a double concave deformity in whichthe lumbar curve is larger and more rigid than the thoracic curve. B: Type II is a double concave deformity in whichthe thoracic curve is more rigid. C: Type III is a thoracic curve. D: Type IV is a long thoracic deformity that tilts intothe curve. E: Type V is a double thoracic curve that tilts into the concavity.

Fig. 11. The definition of complex deformity may be enhancedby the additional use of the scheme proposed by Lenke. Thisscheme emphasizes the CSL. A: The CSL between pedicles up tothe stable vertebra with minimal or no lumbar or scoliosis (Lumbarmodifier A). B: The CSL touches the apical VB or pedicles(Lumbar modifier B). C: The CSL does not touch apical VB orthe VBs immediately above and below the apical disc (Lumbarmodifier C) (arrows denote apical vertebrae).


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Spinal Derotation

Spinal derotation is performed by careful rotation oftwo rods that have been attached to the spine in its de-formed scoliotic state.11 The 90° rotation of the rods canbe used to convert a scoliotic to a kyphotic curve (Fig. 18).If the resultant kyphotic deformity is unacceptable, it maybe corrected by contouring the rod. Biconcave curves mayalso be corrected in this manner. These maneuvers mustbe performed gradually to allow for continuous assess-ment of the implant–bone and component–componentrelationships. If the hooks do not rotate with the rod, sig-nificant stress at the hook–bone interface may occur. Sub-optimally placed pedicle screws may cut out during rodrotation.

Intrinsic Implant Bending Moment Application in theSagittal or Coronal Plane

After placement of screws and loosely connected rods,correction of a scoliotic curvature may be achieved by dis-



Fig. 12. In “bringing the spine to the implant” forces that are oriented along any axis or plane may be used (for exam-ple, the long axis [A], the sagittal plane [B], and the coronal plane [C]). Arrows depict forces applied by the implant.

Fig. 13. Bending moments are applied in the sagittal plane bya three-point bending mechanism (upper left) and an appliedmoment arm cantilever beam mechanism (upper right). Straightarrows depict forces; curved arrows depict bending moments.Lower Left: The three-point bending construct brings the spine tothe implant. Lower Right: The terminal three-point bending con-structs simply have one long and one short moment arm. Straightarrows depict forces applied.

Fig. 14. The crossed-rod technique for correcting thoracic andlumbar kyphotic deformities involving the Harrington distractionrod (A), Luque sublaminar wiring (B), and universal spinal instru-mentation (C). The latter technique is facilitated by the use ofsequential hook insertion (from E.C.B.). The crossed-rod techniquestrategy can be used for coronal-plane (scoliotic) deformities aswell (D). Two rod translation force application strategies can sim-ilarly be used. In this case, a small rod may be applied to the spineand brought to a longer rod that spans the concave side of thedeformity, thus partially correcting the deformity (E).


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traction on the concave side of the curve with simultane-ous distraction on the convex side (Fig. 19 upper left andright). Cross-fixation is usually used to help maintain thecorrection. This technique may be performed to correctboth coronal- and sagittal-plane deformities (Fig. 19 low-er left and right). Care should be taken to achieve fric-tion–glide tightness at the interface prior to distraction so

that the screws maintain their angular relationship with therod during distraction.

Maintenance of Deformity Correction

Cross-fixation of bilaterally placed rods substantiallyaugments the integrity of the construct.12,14,26If the main-tenance of deformity correction depends on cross-connec-tion, as in cases of short-segment parallelogram deformi-ty reduction, the use of cross-fixation in short constructs isessential. Longer constructs involving cross-fixation pro-vide a quadrilateral frame construct that offers increasedrotatory and torsional stability. Cross-fixation also enablesmaintenance of the desired interrod width, which may pre-vent hook migration or screw dislodgment.

Screw triangulation plays an integral role in the preven-tion of lateral translational deformation. Screw toe–in maybe used in conjunction with cross-fixation to allow, in a“belt and suspenders–like” manner, the corrected curva-ture to be maintained.

REGION-SPECIFIC STRATEGIES

The various regions of the spine have unique anatomi-cal and biomechanical properties. Thus, is appropriateto consider correction-related strategies based on the spi-nal region. The CCJ, upper cervical spine, lower cervicalspine, cervicothoracic junction, thoracic spine, thoraco-lumbar junction, lumbar spine, and the lumbosacral regionare each discussed.

Craniocervical Junction and Upper Cervical Spine

The high degree of mobility of the CCJ and upper cer-vical spine leave it vulnerable to deformities in the co-ronal, sagittal, and axial planes. Although many cases maybe corrected by nonoperative means, deformity reduc-tion and occipitocervical fusion are occasionally required.



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Fig. 15. The crossed-rod technique (serially illustrated, A–C)for achieving gradual reduction of a kyphotic deformity.

Fig. 16. Short-segment parallelogram reduction of a lateraltranslational deformity. A: Pedicle screws are placed. B: Thepedicle screws are connected by rods. C: The rods are connected(friction-glide tightness) and a torque applied to both rods simulta-neously by rod grippers. Reduction is achieved and then main-tained using rigid cross-fixation. Distraction, followed by inter-body bone graft placement and compression, is used to secure thebone graft in place.

Fig. 17. A lateral (upper) and axial view (lower) of the crossed-screw fixation technique. Note that rigid cross-fixation maintainsthe near 90° screw toe–in angle.


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Angular bending moment application, which resists ro-tation of this region, may be required. This may be ad-dressed by the techniques of occipitocervical fusion ortransarticular C1–2 screw fixation.

Lower Cervical Spine

Deformity correction in the lower cervical spine pre-sents unique regional challenges. The ease of ventral anddorsal exposure is offset by relatively poor availability ofadequate fixation points, especially dorsally.

Coronal-Plane Deformities. Cervical scoliosis is an un-common entity. The application of concave distractionand convex compression, as well as the use of the derota-tion maneuver, may be used to correct such curves. Therecent introduction of rod-and-screw constructs to cer-vical spine surgery has facilitated the use of these tech-niques.

Sagittal-Plane Deformities. Sagittal-plane deformitiesare relatively common and consist of kyphosis, subsi-dence, and spondylolisthesis.

The degenerative changes of the cervical spine fre-quently alter normal cervical lordosis. This usually beginswith loss of disc space height and follows with subsidenceof the cervical VBs. The moment arm substantially in-creases as kyphosis worsens, and this promotes furtherdeformity. Progression of the deformity may be furthercomplicated by progressive myelopathy.

Ventral, dorsal, or combined approaches can all be usedin cervical kyphosis. A ventral approach provides directdecompression of ventral lesions, while allowing kypho-sis to be corrected by placing a strut graft. A ventral ap-proach can be performed to release the anterior cervicalspine to decrease forces placed during second-stage cor-rection of the dorsal deformity.

Several dorsal techniques, originally developed for thethoracic and lumbar spine, may also be performed to



Fig. 18. A: Spinal derotation is achieved by careful simultaneous rotation of two rods attached to the spine in itsdeformed scoliotic state. B: The rotation of the rods by 90° converts a scoliosis (A) to a kyphosis (B). If the resultantkyphotic deformity is unacceptable, it may be corrected by rod contouring. C and D: This strategy can be applied tobiconcave curves as well.

Fig. 19. Upper: Intrinsic implant bending moment application.Upper Left: In this case, simple distraction of the concave side ofthe curvature and compression of the convex side achieves thereduction of a scoliotic deformity.Upper Right: Cross-fixation isusually used to assist in the maintenance of the reduction.LowerLeft: In this case, laterally placed transverse VB screws are manip-ulated (distracted and compressed; arrows) to reduce a kyphoticdeformity. Lower Right: Compression of the two most dorsalscrews and distraction of the two most ventral screws achievesreduction of this deformity. Cross-fixation is usually used to assistin the maintenance of the reduction.


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reduce cervical kyphotic deformities. The crossed-rodtechnique, enhanced by ventral release, may be conducted(Fig. 20). This can be applied via lateral mass screw fixa-tion. Weakening or disrupting the dorsal tension bandtends to exaggerate sagittal-plane deformations. Fusionmay be indicated in selected patients at risk for post-laminectomy kyphosis. Laminoplasty may decrease therisks of postoperative deformity by minimally disruptingthe dorsal tension band.

Cervical translational deformities are not uncommon.Trauma-induced cervical subluxation may result fromflexion/distraction force applications, which may result infacet dislocation. Closed or open surgical reduction maybe associated with the risk of disrupted disc retropulsioninto the spinal canal and associated neurological sequelae.Many surgeons, therefore, prefer to undertake ventraldecompressive surgery followed by reduction.8,27 Dis-traction of the disc interspace may disengage the lockedfacet joints, allowing for stabilization and fusion in a sin-gle-stage procedure (Fig. 21). Caspar pins may be used toapply rotational forces, to reduce ventrally a unilateraljumped facet. Dorsal reduction and fusion must often be-gin with a partial resection of the facet joint. This iatro-genic destabilization, by removal of the facet joint, mayobligate the incorporation of an additional motion seg-ment into the fusion. Reduction of the deformity and inter-nal fixation complete the procedure. Occasionally, defor-

mity correction cannot be attained via a dorsal approachalone. If neural decompression has been achieved, instru-mentation-assisted fusion in the nonreduced position maybe performed.

In certain cases, a failed initial attempt at ventral reduc-tion may require a combined ventral-dorsal-ventral ap-proach (540°), which provides ventral decompression andboth ventral and dorsal stabilization (Fig. 22).

Cervicothoracic Junction

Biomechanically, the transition from a normal cervicallordosis to thoracic kyphosis makes this region challeng-ing to manage. This is a unique region of the spine. Theshift from the mobile cervical spine to the far less mobilethoracic spine represents a major transition in kinematics.Biomechanical considerations are further complicated bygeometrical, implant–bone interface integrity, and ventralsurgical exposure problems. Decompression of this junc-tion via a dorsal approach may often necessitate dorsalfusion to prevent the development of deformity. Longimplants should not terminate at this junction but shouldextent through the transitional zone (Fig. 23).

Thoracic Spine

The thoracic spine is characterized by larger VBs thatare protected by the rib cage and a relatively smooth bendat each segmental level. Deformities often have elementsin each of the three fundamental planes. Scoliosis is acomplex deformation that is nearly always associated withthe phenomenon of coupling. This occurs when one de-formation along or around an axis obligates a seconddeformation along or around another axis, such as lateralbending and rotation around the long axis of the spine.Thoracic scoliotic deformities rotate the spinous process-es toward the concave side of the curve, resulting in axialload forces transmitted on the concave facet joints. Thisis usually associated with loss of thoracic kyphosis.Correction of the curvature may often require a ventralrelease to achieve adequate correction. Surgical releaseprocedures are usually combined with ventral interbodyfusions.

Coronal-Plane Deformities. Coronal-plane abnormali-ties may be corrected via ventral, dorsal, or combined ap-proaches. In the pediatric population, skeletal maturationmust be a appreciated. In the skeletally immature patient astand-alone dorsal approach may result in unopposed ven-tral growth (crank-shaft phenomenon). Ventral strategiestypically use involve segmental screws and rods placed onthe convex side of the scoliotic curve from neutral verte-bra to neutral vertebra. The result is typically a shorterconstruct than that achieved dorsally. Compression anddistraction, derotation, or a combination of strategies areconducted to reduce thoracic deformities via a ventral ap-proach (Fig. 24). Ventral procedures tend to promote ky-phosis more than dorsal procedures involving constructs.

Dorsal strategies involve similar maneuvers to achievecorrection. Scoliosis correction achieved using dorsal de-rotation methods requires longer constructs and is usuallysupplemented by concave distraction and convex com-pression.9 Longer constructs often lead to a higher inci-dence of rod fracture and acceleration of end-fusion de-



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Fig. 20. The crossed-rod technique of three-point bending forceapplication in the cervical spine. This can be applied by rods andscrews or, as depicted, by rods and wires or cables.


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generative changes. Strategies to combat these includecross-fixation, larger-diameter rods, and external immobi-lization.19 The use of pedicle screw fixation may limit thelength the construct required.

Sagittal Plane Deformities.The first step in correctinga kyphotic deformity is its objective assessment. This maybe accomplished by measuring the angle from the superi-or endplate of the VB one level above the involved VBto the inferior endplate of the VB one level below. Thecorrection itself begins by undertaking the crossed-rodprocedure, which is supplemented with ventral interbody



Fig. 21. The management of a cervical dislocation with locked facet joint(s) via a ventral approach. After a decom-pressive discectomy (A) to release/relax the spine, distraction can be performed using a disc interspace spreader (B). Thisdisengages the locked facet joints. Dorsal rotation and relaxation of the applied forces (after the facets have been“unlocked”) results in the resumption of the normal spine posture (C and D). Fixation and fusion in normal alignmentmay then be achieved (E). Caspar pins and distractors can also be used. Pins placed in an angular orientation can be usedto exaggerate a kyphosis to disengage the facet joints (F), thus permitting reduction (G). Removal of the distractor andpins then restores normal alignment. Rotational deformity, such as that which occurs with a unilateral locked facet, canbe reduced by placing Caspar pins out of the midsagittal plane (H).

Fig. 22. A 540° operation is occasionally indicated. Ventraldecompression (A), followed by a dorsal reduction (B), and ventralstabilization and fusion (C) may be used to decompress, reduce,and stabilize the spine, respectively.

Fig. 23. A: A long implant should perhaps not terminate at thecervicothoracic junction. B: Should this occur, the deformitymay become exaggerated at the terminus of the implant.


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distraction as necessary (Fig. 15). Longer constructs aregenerally more effective because of the larger bendingmoment that can be generated through the longer momentarm. In situ rod bending can also be performed to makefinal adjustments, but this places additional stresses on theimplant, as well as the spine.

Thoracolumbar Junction

The thoracolumbar region is a transitional zone of thespine. The aforementioned strategies for coronal- and sag-ittal-plane deformities in the thoracic region may be simi-larly applied to this region. Ventral release procedures, viaopen or endoscopic techniques, combined with interbodystructural struts, may substantially facilitate deformityreduction and the maintenance of correction.31

Coronal-plane deformities spanning the thoracolumbarjunction often necessitate long dorsal fixation. These de-formities are typically complex, often comprising two ormore curvatures. If the fusion extends to the level of L-4and below, motion will be significantly affected. Coronal-plane balance, which may be assessed by using the CSL,must not be overlooked.

Degenerative changes of the spine may be related to thedevelopment and evolution of scoliotic curves in adults. Incases of scoliosis in which a progressive deformity devel-ops late in life, management may be quite difficult. De-generative facet joint and disc disease are always present,

and the curves are associated with a loss of lumbar lordo-sis. The aging spine with concurrent osteoporosis may sig-nificantly limit surgical options. Predictors of deformityprogression include lateral spondylosis of the apical verte-bra, a Cobb angle of 30° or more, the number of vertebraein the curve, degree of disc wedging within the curve (discindex), lateral vertebral translation of 6 mm or more, andthe prominence of L-5 in relation to the intercrest line.

Lumbar Spine

The aforementioned strategies for the thoracic and tho-racolumbar spine are likewise applicable to the lumbarspine. The majority of deformities have a significanttranslation component that requires careful considerationof sagittal-plane balance. An important factor in the at-tainment and maintenance of lordosis is intraoperativepositioning. Surgical beds or frames that facilitate lordosisby encouraging extension of the spine and hips are opti-mal. Intraoperative pelvic flexion can result in inadequatelordosis with a resultant flat back. The sagittal vertebralaxis should be brought back to the dorsal L5–S2 joint,which may require aggressive osteotomy and/or ventralload-bearing adjuncts.



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Fig. 24. Coronal-plane deformities may be reduced using com-pression and distraction (A), the crossed-rod technique (B), thederotation maneuver (C), or a combination of these techniques.

Fig. 25. Upper: Long moment arms (d and d�) that pass ven-tral or caudal to the lumbosacral pivot point (dot) can apply ade-quate leverage for deformity correction and prevention. An L5–S1spondylolisthesis (center left) can be managed by performing anL-5 corpectomy (center right) and a reduction and the docking ofL-4 on S-1 (lower left). An interbody fusion may be used as aspacer and for fusion acquisition. Dorsal instrumentation maintainsfixation (lower right). Care must be taken to ensure that adequateroom is provided for both nerve roots at the new L4–S1 juncture.


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Lumbosacral Region

The lumbosacral junction poses unique and complexbiomechanical challenges. Instability may be assessed byexamining flexion and extension radiographs. Whereasthey are commonly obtained while the patient is standing,the lateral decubitus position off-loads the spine, thus min-imizing pain and allowing a more accurate assessment.Spondylolisthesis may require aggressive surgical strate-gies and accompanying lumbar, sacral, and pelvic fix-ation. The likelihood of deformity progression must beknown. Patients in whom the degenerative process hasbegun are less likely to slip further than those in whom ithas yet to begin. Partial correction is often obtained afterpositioning. Three-point bending force application mayfacilitate reduction by pulling back the intermediate (L-5)screw of a three-screw construct. Long moment arms that

pass ventral or caudal to the lumbosacral pivot point areoften required to achieve adequate correctional bendingmoments (Fig. 25 upper). Although complete deformitycorrection is the goal, it is often difficult and unnecessary,even when decompression in performed.

In cases of L5–S1 spondylolisthesis, the L-5 VB mayhave to be removed and L-4 fused to the sacrum by usingan intervening strut graft that bears axial loads. The use ofinterbody morselized bone graft may be associated with ahigher incidence of pseudarthrosis. Dorsal instrumenta-tion maintains reduction (Fig. 25 center and lower).

Ventral and Dorsal Osteotomy

A variety of ventral and dorsal osteotomy types mayhelp to achieve deformity correction. The risks of neuralinjury must be weighed against the potential benefits ofthe osteotomy-induced correction. When considering theusefulness of osteotomy, two factors are important: extentof correction required and degree of ankylosis. The axisaround which the correction is to be achieved must beconsidered (Fig. 26). The goal of deformity correction viaosteotomy is to shift the sagittal vertebral axis dorsally,bringing the spine into balance.

Regardless of the type of osteotomy performed, it ismost effective when performed at the apex of a curve.Dorsal osteotomy is most safely performed in the lumbarpine, whereby the disc interspace and pedicles have beenremoved. The axis of rotation is around the anterior longi-tudinal ligament (Fig. 27 left). The egg-shell osteotomy isa variant of the dorsal osteotomy. This technique involvesresection of the pedicle via a dorsal approach and subcor-tical resection of VB medullary (cancellous) bone. Thisfacilitates collapse of the VB in a wedgelike manner (Fig.27 center and right).

CONCLUSIONS

The correction of deformity may be achieved by a vari-ety of methods, each with advantages and disadvantages.Understanding the biomechanical principles involvedfacilitates the clinical decision-making process, thus en-abling the surgeon to optimize patient outcome. Theultimate goal is to ensure a biomechanically sound envi-ronment, and facilitate a nonpathological relationship be-tween the neural elements and the surrounding osseousand soft tissue confines.



Fig. 26. The axis for sagittal-plane correction is perpendicularto the long axis of the spinal axis (dot in the lateral view). This axismay be located in the region of the spinal canal (A). It may also belocated ventrally, in the region of the anterior longitudinal ligament(for example, for dorsal wedge osteotomies [B] or in the middlecolumn region [C]).

Fig. 27. Left: Dorsal osteotomy. Center and Right: Egg-shell osteotomy.


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Manuscript received November 29, 2002.Accepted in final form December 11, 2002.Address reprint requests to: Edward C. Benzel, M.D., Director,

Spinal Disorders, Cleveland Clinic Foundation, Department ofNeurosurgery/S80, 9500 Euclid Avenue, Cleveland, Ohio 44195.



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